# std::is_unsigned

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Defined in header `<type_traits>` |  |
template< class T > struct is_unsigned; |  | (since C++11)

`std::is_unsigned` is a UnaryTypeTrait.

Checks whether `T` is an unsigned arithmetic type.

- If std::is_arithmetic<T>::value is true, provides the member constant `value` equal to T(0) < T(-1).
- Otherwise, provides the member constant `value` equal to false.

If the program adds specializations for `std::is_unsigned` or `std::is_unsigned_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_unsigned_v = is_unsigned<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is an unsigned integral type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    namespace detail
    {
        template<typename T,bool = std::is_arithmetic<T>::value>
        struct is_unsigned : std::integral_constant<bool, T(0) < T(-1)> {};

        template<typename T>
        struct is_unsigned<T,false> : std::false_type {};
    } // namespace detail

    template<typename T>
    struct is_unsigned : detail::is_unsigned<T>::type {};

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    class A {};
    static_assert(std::is_unsigned_v<A> == false);

    enum B : unsigned {};
    static_assert(std::is_unsigned_v<B> == false);

    enum class C : unsigned {};
    static_assert(std::is_unsigned_v<C> == false);

    struct S { unsigned p : 1; int q : 1; };
    static_assert
    (
        std::is_unsigned_v<decltype(S::p)> not_eq
        std::is_unsigned_v<decltype(S::q)>
    );

    static_assert
    (
        std::is_unsigned_v<float> == false &&
        std::is_unsigned_v<signed int> == false &&
        std::is_unsigned_v<unsigned int> == true &&
        std::is_unsigned_v<bool> == true
    );

    int main()
    {
        // signedness of char is implementation-defined:
        std::cout << std::boolalpha << std::is_unsigned<char>::value << '\n';
    }

Possible output:

    false

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2197 | C++11 | `value` could be true even if `T` is not an arithmetic type | can only be false in this case

### See also

is_signed (C++11) | checks if a type is a signed arithmetic type (class template)
is_signed [static] | identifies signed types (public static member constant of `std::numeric_limits<T>`)
is_arithmetic (C++11) | checks if a type is an arithmetic type (class template)
make_signed (C++11) | obtains the corresponding signed type for the given integral type (class template)
make_unsigned (C++11) | obtains the corresponding signed type for the given integral type (class template)

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